Fix headings in wisun_intro.md

Reduce heading levels, so page doesn't attach to page before.

Author: Amanda Butler

docs/reference/technology/mesh/wisun_intro.md

@@ -1,4 +1,4 @@
-#### Wi-SUN
+## Wi-SUN
 
 The Wi-SUN Field Area Network (FAN) is based on open standards from IEEE802, IETF, ANSI/TIA and ETSI.
 
@@ -8,29 +8,29 @@ The Mbed OS Wi-SUN stack is built on IPv6 over Low power Wireless Personal Area
 
 You can find more details about Wi-SUN at the [Wi-SUN Alliance site](https://www.wi-sun.org).
 
-#### Mbed Wi-SUN API
+### Mbed Wi-SUN API
 
 Mbed OS provides the [Mesh C++ API](../apis/mesh-api.html) for building Wi-SUN applications.
 
 - To connect to the Wi-SUN network, use the [Wi-SUN interface API](https://github.com/ARMmbed/mbed-os/blob/master/features/nanostack/mbed-mesh-api/mbed-mesh-api/WisunInterface.h).
 - For the socket communication over the Wi-SUN network, use the [Mbed sockets API](../apis/network-socket.html).
 
-##### Nanostack Wi-SUN API
+#### Nanostack Wi-SUN API
 
 Mbed Wi-SUN is implemented in the Nanostack library that provides a set of C API headers with more functionalities. The [nanostack folder](https://github.com/ARMmbed/mbed-os/tree/master/features/nanostack/sal-stack-nanostack/nanostack) has the following header files that can be used with Wi-SUN:
 
 - `net_interface.h` for initializing, starting and stopping a Wi-SUN interface.
 - `ws_management_api.h` for configuring device behavior.
 - `ws_bbr_api.h` for Wi-SUN backbone border router application interface.
 
-#### Mbed Wi-SUN examples
+### Mbed Wi-SUN examples
 
 Mbed Wi-SUN network consists of routers and border router(s). Mbed OS provides example applications to build Wi-SUN application.
 
-##### Router
+#### Router
 
 You can find an example using the [Mesh C++ API](../apis/mesh-api.html) for building a Wi-SUN router device in the [mesh minimal example](https://github.com/ARMmbed/mbed-os-example-mesh-minimal).
 
-##### Border router
+#### Border router
 
 You can find an example using the [Nanostack API](https://github.com/ARMmbed/mbed-os/tree/master/features/nanostack/sal-stack-nanostack/nanostack) to biuld a Wi-SUN border router at [nanostack border router](https://github.com/ARMmbed/nanostack-border-router).